Significant transitions of microstructure and mechanical properties in laser additive manufacturing AlCoCrFeNi2.1 eutectic high-entropy alloy under heat treatment

Abstract

In this work, the AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) were fabricated by laser additive manufacturing and subsequently heat-treated at various temperatures between 800-1200°C to achieve excellent strength-plasticity synergy. The high solidification rate of selective laser melting (SLM) leads to instability of the eutectic growth front, resulting in a typical melt pool morphology of equiaxed and columnar crystals for AlCoCrFeNi2.1. The body-centered cubic (BCC) content of ∼82% in the SLM specimens leads to a higher tensile strength of ∼1175 MPa and poorer plasticity of ∼6.42%. After heat treatment, the BCC transforms to face-centered cubic (FCC) and the microstructure changes to a dendrite containing eutectic lamellar structure. As the temperature increases, Ni-Al aggregates, B2 phase particles precipitate and grow in the BCC matrix. Meanwhile, the uniform heating mode eliminates the orientation of the SLM melt pool structure, reduces the internal stress and dislocation density of the SLM alloy. In addition, high-temperature heating promotes the stable growth of lamellar structure. These combined effects allow the strength-plasticity trade-off to be achieved in the heat-treated specimen. The HT-1200 sample has a strength of 1065MPa and a ductility of 21.97%.